How to: upgrade a 520/540 to a 550c ROM

[croissantking]

So the full range 0x40000000 to 0x403FFFFF might already be addressable. I could probably mock together a 4MB ROM file for you that has an embedded ROM disk driver in the first 2MB, and the ROM disk in the second 2MB. I won't really have a way to test it, but you could try burning it on your prototype whenever you get around to it. Want to give this a try?

I didn't see this part of your message – yes, sure go ahead! Although it's a bit fiddly, I dont mind soldering + desoldering a few times to get this right.

I'm still keen to understand why I'm not able to dump past the 2mb limit on my own machine...

 

[frontein1]

Oh, wow... I was shortly going to post that I couldn't get my PowerBook to read past the 2MB boundary!

I see that on yours, 40200000 is displaying the same contents as 40000000 ... not sure what that means in practice but it seems to be looping back to the first 0-2MB in the 2-4MB part of the map.

Very interesting difference. Yes on mine, the ROM is essentially being aliased over again from 40200000 to 403FFFFF. But not on your machine. (Although to be fair I am not totally convinced if this is true aliasing or if this space is addressable. A good test would be that custom 4MB ROM and then dumping 0x40200000 to make sure that is different from 0x40000000.)

It might be interesting for you to issue this in the debugger: DM 40080870. What does that look like? Here is the one from my 540c (Just to rule out MacsBug I switched back to MicroBug and was still able to dump out the second 2MB area):

 

[croissantking]

I am very confused, Mr Frontein1!

I tried DM 40080870 and it spat out the same data as on yours, but DM 40200000 still told me it couldn't access that address.
So I pulled out another 540c (different machine) and it displays the aliased ROM at 40200000, just like yours.
Then I removed the CPU card from the first machine which has a 550c ROM, and installed another with the 520/540 ROM. It displayed the aliased data just like yours.
Then I put back the card with 550c ROM and now it is showing the actual contents of the ROM past the 2MB mark!

 

[demik]

What are you guys CPU board revisions ? AFAIK we still don't know the differences, might be one of them

 

[croissantking]

What are you guys CPU board revisions ? AFAIK we still don't know the differences, might be one of them

The main machine I'm using for testing is an 820-0456-B, the very first one.

 

[croissantking]

Just sharing the ROM chip pin assignments that I've mapped out in case it's useful.

@frontein1 what's the maximum size ROM that could be addressed with the address lines available?

Interesting how the data lines are interleaved.

ROM Low (U706)		ROM High (U708)
Pin	Assignment	Pin	Assignment
1	N.C.	1	N.C.
2	A20	2	A20
3	A19	3	A19
4	A9	4	A9
5	A8	5	A8
6	A7	6	A7
7	A6	7	A6
8	A5	8	A5
9	A4	9	A4
10	A3	10	A3
11	A2	11	A2
12	CE	12	CE
13	GND	13	GND
14	OE	14	OE
15	D0	15	D16
16	D8	16	D24
17	D1	17	D17
18	D9	18	D25
19	D2	19	D18
20	D10	20	D26
21	D3	21	D19
22	D11	22	D27
23	VCC	23	VCC
24	D4	24	D20
25	D12	25	D28
26	D5	26	D21
27	D13	27	D29
28	D6	28	D22
29	D14	29	D30
30	D7	30	D23
31	D15	31	D31
32	GND	32	GND
33	VCC	33	VCC
34	A18	34	A18
35	A17	35	A17
36	A16	36	A16
37	A15	37	A15
38	A14	38	A14
39	A13	39	A13
40	A12	40	A12
41	A11	41	A11
42	A10	42	A10
43	A21	43	A21
44	N.C.	44	N.C.